The present invention relates to a four wheel steering system for steering front wheels and rear wheels, and more specifically to a steering system having two hydraulic actuators for steering the front wheels and the rear wheels, respectively.
Various four wheel steering systems are shown in patent documents. Some examples are U.S. Pat. Nos. 4,666,013; 4,705,131 and 4,679,809, and commonly assigned, copending United States patent applications Ser. Nos. 07/195,085 (filed on May 17, 1988), 07/269,698 (filed on Nov. 10, 1988), 07/305,023 (filed on Feb. 2, 1989), and 07/314,874 (filed on Feb. 24, 1989).
A four wheel steering system of the abovementioned U.S. Pat. No. 4,666,013 has a front wheel auxiliary steering system and a rear wheel auxiliary steering system which are both of a hydraulic type. Both of the front and rear wheel auxiliary steering systems receive hydraulic fluid pressure from a common fluid pressure source, and each system has a servo valve, and a steer angle sensor for feedback control.
This four wheel steering system of such a closed center type is superior in control accuracy, but disadvantageous in delay of response due to the feedback control, and in the manufacturing cost raised by the servo valves which are expensive, and the addition of the sensors for sensing the auxiliary steer angles.
Therefore, some four wheel steering systems utilize two separate fluid pressure sources, and pressure control valves which are not the servo type. However, this arrangement requires two oil pumps, and therefore, increases the cost, and the size of the system.